The present invention relates to an overtape-sealed paper bag used as a heavy-duty paper shipping bag.
The machine-sewed heavy-duty paper shipping bag (referred to hereinafter simply as "sewn bottom bag") of prior art comprises a paper tube having opposite open ends covered with respective twofold tapes and sealed by machine-sewing said tapes integrally with the paper tube itself. In view of its high economical efficiency, it has conventionally been considered as unavoidably allowable drawback that such machine-sewed bag is more or less moisture absorbable and prone to rupture. However, a recent tendency to require strict quality guarantee of the content has made such moisture absorbability and bag rupture a problem to be solved and accordingly this problem has presently been solved by overtaping the open ends of the paper tube so as to cover stitching-thread as well as sewing perforations.
For the machine-sewed bag of well known art, the side down posture drop test from a height of 1.2 m indicated that the bag ruptures along the sewed lines, i.e., opposite sides on an average after twice of such drop test.
The machine-sewed bag of prior art which has been overtaped as mentioned above certainly reduces the moisture absorbability and possibility of bag rupture but a readiness for unsealing, one of the most important conveniences to be provided by the machine-sewed bag is thereby vitiated and use of the overtape necessarily increases the cost of the bag. In the case of a moistureproof stitched bag which contains therein a separate bag made of synthetic resin film, the bottom of the inner tube must be heat sealed above the level at which the outer tube will be machine-sewed, since the inner and outer tubes are simultaneously manufactured. This results in a corresponding tube loss (at least 60 mm collectively at top and bottom open ends). Even so far as only the outer tube is concerned, machine-sewing treatment requires margins of approximately 15 mm at top and bottom open ends, respectively. Accordingly, a length of the paper tube for the outer bag must be substantially increased relative to a filling capacity and no material reduction can be achieved.
Furthermore, a paper bag having its bottom open end covered and sealed with a twofold bottom sealing paper is disclosed by Japanese Utility Model Publication No. 1975-3608 whose main object is to provide improved sealing at its bottom open end, not to provide readiness for unsealing.
A paper bag having its bottom open end covered and sealed with a twofold bottom sealing paper sheet which contains, in turn, a tear strip extending horizontally is also known from, for example, Japanese Utility Model Publication No. 1969-7819; Japanese Utility Model Disclosure Gazette No. 1979-106911; and Japanese Utility Model Disclosure Gazette No. 1989-20451. However, a readiness for unsealing depends on a cutting resistance and cutting of the bottom sealing paper sheet and so unsealing of the bag by pulling the tear strip contained therein can not be smoothly done.
As the well known construction closely related to the heavy-duty paper shipping bag of the invention, a bag made of synthetic resin woven fabric having its bottom open end covered and sealed with a twofold synthetic resin tape by use of hot melt adhesive which has a melting point lower than that of said synthetic resin is disclosed by Japanese Utility Model Publication No. 1969-15573. However, such bag of prior art has been found to be extremely poor in its readiness for unsealing.
A similar bag which is made of synthetic resin and has its bottom open end sealed with a twofold synthetic resin tape is disclosed by Japanese Utility Model Publication No. 1966-18387 and Japanese Utility Model Disclosure Gazette No. 1972-8523. However, both of these well known bags have been also found to be very poor in their readiness for unsealing.